#include <bits/stdc++.h>
#define f first
#define s second
#define ll long long
using namespace std;
const int MAXN = 3e4+10;
vector<pair<int,ll>> adj[MAXN];
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int N,M; cin >> N >> M;
vector<pair<int,int>> doges(M);
for(int i = 0;i<M;++i){
cin >> doges[i].f >> doges[i].s;
}
for(int i = 0;i<M;++i){
for(int j = i+1;j<M;++j){
if(doges[j].f == doges[i].f){
adj[i].push_back({j,0});
adj[j].push_back({i,0});
//cout << i << " " << j << "\n";
//cout << j << " " << i << "\n";
continue;
}
if((abs(doges[j].f-doges[i].f)) % doges[i].s == 0){
//cout << j << " " << i << "\n"; //j to i
adj[j].push_back({i,abs(doges[j].f-doges[i].f)/doges[i].s}); //start from the target (1), the directed edges are directed towards 0
}
if((abs(doges[j].f-doges[i].f)) % doges[j].s == 0){
//cout << i << " " << j << "\n"; //i to j
adj[i].push_back({j,abs(doges[j].f-doges[i].f)/doges[j].s});
}
}
}
priority_queue<pair<ll,int>> pq; pq.push({0,1});
vector<ll> dist(M,-1); dist[1] = 0;
while(!pq.empty()){
int u = pq.top().s; ll cd = pq.top().f; pq.pop();
if(cd != dist[u]) continue;
for(auto p : adj[u]){
if(cd+p.s < dist[p.f] || dist[p.f] == -1){
dist[p.f] = cd+p.s;
pq.push({dist[p.f],p.f});
}
}
}
cout << dist[0] << "\n";
return 0;
}
